1. Field of the Invention
The present invention relates to a light emitting device using an element (hereinafter referred to as xe2x80x9can organic EL elementxe2x80x9d) which has an anode layer, a cathode layer, and a film (hereinafter referred to as xe2x80x9can organic EL layerxe2x80x9d) including an organic compound in which an EL (electro luminescence; luminescence produced by applying an electric field) is produced. As the EL in the organic compound, there are light emission (fluorescence) generated in returning from a singlet excitation state to a ground state and light emission (phosphorescence) generated in returning from a triplet excitation state to a ground state. The present invention particularly relates to a light emitting device in which a metal complex, which is capable of forming pores due to a two dimensional or a three dimensional mesh structure, is applied to a light emitting layer and thus light emitting materials are arranged in the pores to promote light emission of phosphorescence. Note that a light emitting device in this specification indicates an image display device or a light emitting device using an organic EL element as a light emitting element. Also, a module in which a TAB (tape automated bonding) tape or a TCP (tape carrier package) is attached to the organic EL element, a module in which a printed wiring board is provided in the end of the TAB tape or the TCP, and a module in which an IC (integrated circuit) is directly mounted on the organic EL element by a COG (chip on glass) method are all included in the light emitting device.
2. Description of the Related Art
An organic EL element is an element which emits light by applying an electric field thereto. According its light emitting mechanism, a voltage is applied to an organic EL layer sandwiched between electrodes and thus an electron injected from a cathode and a hole injected from an anode are recombined in a light emitting center of the organic EL layer to form a molecule with an excitation state (hereinafter referred to as xe2x80x9ca molecular excitonxe2x80x9d) and the molecular exciton releases energy to emit light in returning to a ground state.
In the general organic EL element, the organic EL layer is made from a thin film which is thinner than 1 xcexcm. Also, since the organic EL element itself is a self light emission type, a back light as used in a conventional liquid crystal display is not required. Therefore, it is a great advantage that the organic EL element is manufactured to be extremely thin and light weight.
Also, in the case of the organic EL layer with, for example, about 100 to 200 nm, when the carrier mobility of the organic EL layer is considered, a period from the injection of carriers to the recombination is about several tens of nanoseconds. Even if a process from the recombination to the light emission is included in the period, light emission is carried out within an order of microseconds. Therefore, a very high response speed is one of the characteristics.
Further, since the organic EL element is a carrier injection type light emitting element, it can be driven by a direct current voltage and a noise is hard to cause. With respect to a drive voltage, drive is allowed at an order of several volts by a method of selecting an electrode material in which a carrier injection barrier is lowered, a method of introducing a heterostructure (laminate structure), or the like (Reference 1: C. W. Tang and S. A. VanSlyke, xe2x80x9cOrganic electroluminescent diodesxe2x80x9d, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)). In Reference 1, an alloy of Mg:Ag is used as a cathode and a hetero structure in which a diamine compound and tris(8-quinolinolato) aluminum (hereinafter referred to as xe2x80x9cAlq3xe2x80x9d) are laminated is used, and thus direct current low voltage drive is realized.
Because of the above characteristics such as a thin type, light weight, fast response, and direct current low voltage drive, the organic EL element attracts an attention as a next generation flat panel display element. Also, the organic EL element is a self light emission type and has a wide view angle. Thus, its visibility is relatively high and it is considered that the organic EL element is effective as an element used for a display screen of mobile equipment.
Here, as described above, the organic EL is a phenomenon in which light is emitted when the molecular exciton is returned to the ground state, and a singlet excitation state (S*) and a triplet excitation state (T*) are allowed as the excitation state of the molecular exciton produced by the organic compound. Also, it is considered that the statistical generation ratio in the organic EL element is S*:T*=1:3 (Reference 2: Tetsuo Tsutsui, xe2x80x9cThe Japan Society of Applied Physics, Organic Molecule and Bioelectronics Division, Third Seminar Textxe2x80x9d, p.31 (1993)).
However, with respect to a general organic compound, light emission (phosphorescence) from the triplet excitation state (T*) is not observed at a room temperature and only light emission (fluorescence) from the singlet excitation state (S*) is generally observed. This is because the ground state of the organic compound is generally the singlet excitation state (So) and thus a transition T*xe2x86x92So becomes a forbidden transition and a transition S*xe2x86x92So becomes an allowed transition.
That is, only the singlet excitation state (S*) generally contributes light emission and this is the same in the case of the organic EL element. Thus, it is assumed that a theoretical limitation of internal quantum efficiency (ratio of photons generated to injected carriers) in the organic EL element is 25% based on evidence of S*:T*=1:3.
Also, all generated light is not emitted to the outside and a part of the light cannot be picked up due to refractive indexes which are inherent to organic EL element constituent materials (organic EL layer material and electrode material) and a substrate material. A ratio of the light picked up toward the outside to the generated light is called light pickup efficiency. It is said that the pickup efficiency in the organic EL element which has a glass substrate is about 20%.
From the above reason, even if all the injected carriers form the molecular excitons, it is said that the theoretical limitation of a ratio of photons finally picked up toward the outside to the number of injected carriers (hereinafter referred to as xe2x80x9cexternal quantum efficiencyxe2x80x9d) is 25%xc3x9720%=5%. That is, even if all carriers are recombined, only 5% of the recombined carriers are picked up as light according to calculation.
However, recently, organic EL elements, whish are capable of converting energy released in returning from a triplet excitation state (T*) to a ground state (hereinafter referred to as xe2x80x9ctriplet excitation energyxe2x80x9d) into light to be emitted, are successively reported and their high light emission efficiencies are noted (Reference 3: D. F. O""Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, xe2x80x9cimproved energy transfer in electrophosphorescent devicesxe2x80x9d, Applied Physics Letters, Vol. 74, No. 3, 442-444 (1999) and Reference 4: Tetsuo Tsutsui, Moon-Jae Yang, Masayuki Yahiro, Kenji Nakamura, Teruichi Watanabe, Taishi Tsuji, Yoshinori Fukuda, Takeo Wakimoto and Satoshi Miyaguchi, xe2x80x9cHigh Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center,xe2x80x9d Japanese Journal of Applied Physics, Vol. 38, L1502-L1504 (1999)).
In Reference 3, a metal complex with platinum as main metal (hereinafter referred to as xe2x80x9ca platinum complexxe2x80x9d) is used. Also, in Reference 4, a metal complex with iridium as main metal (hereinafter referred to as xe2x80x9can iridium complexxe2x80x9d) is used. Thus, it can be said that it is a characteristic to introduce a third transition series element as main metal in any metal complex. There is an organic EL element in which the theoretical limitation value of the external quantum efficiency as described above greatly exceeds 5%.
As described in References 3 and 4, with respect to the organic EL element which is capable of converting the triplet excitation energy into light to be emitted, higher external quantum efficiency than a conventional element can be achieved. And, if the external quantum efficiency is increased, a light emission intensity is improved. Thus, it is considered that the organic EL element, which is capable of converting the triplet excitation energy into light to be emitted, has a large share in future developments as a manner for achieving high intensity light emission and high light emission efficiency.
However, since both platinum and iridium are so-called noble metal, the platinum complex and the iridium complex using these metals are expensive and thus it is expected that a cost reduction is hindered in future.
In addition, a color of light which the above iridium complex emits is a green color, that is, a wavelength located in the middle of a visible light region. When the platinum complex is used as a dopant, it emits light of a red color with a relatively high color purity. However, there are the following defects. That is, in the case of the platinum complex with a low concentration, a color purity is decreased since a host material also emits light. In the case of a high concentration, light emission efficiency is reduced because of concentration quench. In other words, high efficiency light emission of a red color and a blue color, which have a high color purity, is not obtained from the organic EL element which is capable of converting the triplet excitation energy into light to be emitted.
Therefore, when it is considered that a full color flat panel display is manufactured using light emission colors of red, green, and blue in the future, it is necessary to achieve red color light emission and blue color light emission, which have high external quantum efficiency and a high color purity in the cases of the iridium complex and the platinum complex, using a lower cost material as much as possible.
From such a background, it is desirable that the organic EL element which is capable of converting the triplet excitation energy into light to be emitted, except the organic EL element using the iridium complex or the platinum complex, which already exists, is developed. A most simple method is developing a new organic compound in which light is emitted as phosphorescence at a room temperature. However, a clear molecular design plan is not established until now and is very difficult in many aspects.
Thus, although it is important to develop a new material which emits phosphorescence light, a method of designing a structure of the organic EL layer, in which phosphorescence light emission is promoted, is desirable for light emitting materials used in a conventional organic EL element. This reason is as follows. That is, since various light emission colors have been already obtained in the case of the light emitting materials used in the conventional organic EL element, there is a possibility that various light emission colors are obtained and a large number of low cost materials are present.
Therefore, an object of the present invention is to obtain an organic EL element which is capable of converting triplet excitation energy into light to be emitted by devising a structure of a light emitting layer with using a light emitting material used in a conventional organic EL element. Thus, an object of the present invention is to provide an organic EL element which has high light emission efficiency, indicates various light emission colors by using a conventional organic compound, and can be manufactured at a low cost.
Also, an object of the present invention is to provide a light emitting device at a low cost, which is light, has low consumption power, and indicates various light emission colors, using an organic EL element disclosed by the present invention. Further, an object of the present invention is to provide an electronic device which is light, has low consumption power, indicates various light emission colors, and which has a low cost by using such a light emitting device.
The present inventor focused attention on a heavy atom effect which is known in the field of PL (photo luminescence; luminescence produced by light irradiation). The heavy atom effect is that spin-orbit interaction is increased and light emission of phosphorescence is promoted when a heavy atom (atom which has a large number of atomic nucleus loads) is introduced into a molecular or a solvent. Note that the atomic nucleus load corresponds to an atomic number, that is, the number of positive electric charge of an atomic nucleus.
The heavy atom effect includes two types, that is, an internal heavy atom effect and an external heavy atom effect. The internal heavy atom effect is that light emission of phosphorescence is promoted in the case where a heavy atom is included in a molecular of a light emitting material. On the other hand, even when a heavy atom is present in a solvent containing a light emitting material as a solute, there is the case where the promotion of phosphorescence light emission of the light emitting material is observed. This phenomenon is called the external heavy atom effect.
Thus, if the external heavy atom effect can be produced even in the organic EL element, it is considered that there is a possibility of generating phosphorescence. That is, this is a method of locating a material including a heavy atom near the light emitting material to promote the phosphorescence light emission.
Simply, although a method of dispersing metal as a heavy atom into the light emitting layer of an organic EL layer is considered, it is hard to function as the light emitting layer in this case. For example, when alkali metal (such as cesium) is doped into the organic EL layer, the electrical conductivity of the doped layer is improved and the layer can exert a superior function as a carrier transport layer. However, since the doped metal becomes a material for deactivating excitation energy to suppress light emission (hereinafter referred to as xe2x80x9ca quencherxe2x80x9d), the doped layer does not emit light. Therefore, it is generally difficult to use the doped layer as the light emitting layer and the introduction of the heavy atom effect becomes impossible.
Thus, it is essential that not metal itself but an insulator including a heavy atom is located near the light emitting material. That is, when an insulator with a large band gap is used as a material including the heavy atom, energy transfer from the light emitting material and deactivation are suppressed and it is prevented that the material becomes a quencher. Therefore, the promotion of the phosphorescence light emission by the heavy atom effect can be expected.
For example, there is a report with respect to PL characteristic of an organic material which is dispersed into pores of zeolite which is an insulator (Reference 5: V. Ramamurthy, J. V. Caspar, D. F. Eaton, Erica W. Kuo, and D. R. Corbin, xe2x80x9cHeavy-Atom-Induced Phosphorescence of Aromatics and Olefins Included within Zeolitesxe2x80x9d, Journal of American Chemical Society, Vol. 114, No. 10, 3882-3892 (1992)). According to Reference 5, with respect to the PL of the organic material included in the pores of the zeolite, when cations (Li, Na, K, Rb, Cs, and TI) of the zeolite are substituted in succession, the phosphorescence light emission is promoted by the heavy atom effect as the cation becomes a heavier atom is obtained.
Here, in particular, the present inventor considers that a method of using the insulator including the heavy atom in a form of a porous body (in which a distribution of a pore size is uniform if possible) and introducing the light emitting material to the insides of the pores of the porous body is desirable as in Reference 5. This is because a structure in which the light emitting material is surrounded by the porous body is obtained, and thus it is expected that the interaction between the heavy atom included in the porous body and the light emitting material is increased and the promotion of phosphorescence light emission by the heavy atom effect is easy to be produced. Also, since the light emitting material is trapped in the pores and the pores are systematically arranged, this becomes a state in which a pseudo superlattice structure and thus there is a possibility that a stable molecular exciton is produced and the improvement of a light emitting characteristic is led.
Note that there are several problems in the case where the light emitting material is included in the zeolite to drive the organic EL element. First, when the zeolite is provided on an electrode, there is a possibility that the injection of carrier into the organic EL layer is prevented and the light emitting characteristic is deteriorated. Also, a technique for forming the zeolite as a thin film with a thickness of about 100 to 200 nm is required. Further, in the case of vacuum evaporation, the light emitting material cannot be included in the zeolite (it is allowed in a dry method).
When these problems with respect to an electrical characteristic and a process are considered, it is said that the method of promoting the phosphorescence light emission using the zeolite, as described in Reference 5, is easy because of the PL. That is, it is relatively difficult to apply this method to the EL element.
Thus, the present inventor devised a method, in which a concept that the light emitting material is included in the pores of the porous body including the heavy atom is utilized, and which is hard to cause the problems with respect to the electrical characteristic and the process. The method uses a metal complex which has a mesh-shaped structure as a host to the light emitting material and disperses the light emitting material into the mesh of the host. Its schematic view is shown in FIGS. 1A and 1B.
As the mesh formed by the metal complex (hereinafter referred to as merely xe2x80x9ca latticexe2x80x9d), a form as shown in FIG. 1A in which metal atoms 101a are located in lattice points and cross-linked through ligands 102a (hereinafter referred to as xe2x80x9ca lattice A1xe2x80x9d) and a form as shown in FIG. 1B in which ligands 102b are located in lattice points and cross-linked through metal atoms 101b (hereinafter referred to as xe2x80x9ca lattice B1xe2x80x9d) are considered. Note that, it is considered that a shape of the lattice is not limited to a quadrangle as shown in FIGS. 1A and 1B and various a polygon (such as a hexagon) can be used. Also, both a two dimensional (plane) structure and a three dimensional (solid) structure can be used for the lattice.
Then, when the metal complex which has such a lattice structure is mixed with light emitting material 103 which has a suitable molecular size at the synthesis or at the film formation, the light emitting material can be introduced in the positions between the lattices of the lattice structure.
Thus, according to the present invention, a light emitting device which has an organic EL element including a light emitting layer made of an organic compound in which an EL is obtained and a metal complex, is characterized in that the metal complex has a lattice structure in which metal atoms and ligands are alternately arranged, and the lattice structure has a structure that the metal atoms are located in lattice points and the lattice points are cross-linked through the ligands (lattice A1).
Also, according to the present invention, a light emitting device which has an organic EL element including a light emitting layer made of an organic compound in which an EL is obtained and a metal complex, is characterized in that the metal complex has a lattice structure in which metal atoms and ligands are alternately arranged, and the lattice structure has a structure that the ligands are located in lattice points and the lattice points are cross-linked through the metal atoms (lattice B1).
Note that, it is considered that most metal atoms can produce the heavy atom effect in different degrees. In the field of PL, the heavy atom effect is clearly produced in the case where the metal complex includes an atom with a weight equal to ro larger than particularly bromine (Br; atomic number is 35). Therefore, a metal atom which is heavier than rubidium (Rb; atomic number is 37) is preferable as a metal atom included in the metal complex.
Thus, according to the present invention, a light emitting device which has an organic EL element including a light emitting layer made of an organic compound in which an EL is obtained and a metal complex, is characterized in that the metal complex has a lattice structure of the lattice A1 or the lattice B1, in which metal atoms and ligands are alternately arranged and the metal atoms have an atomic number equal to or larger than rubidium.
Note that, the heavy atom effect is that a heavy atom is introduced to increase spin-orbit interaction and to promote phosphorescence light emission. Thus, even in the case where the heavy atom is not used, the triplet excitation energy can be converted into light to be emitted if a molecular structure with large spin-orbit interaction can be introduced.
As one method, it is considered to introduce a molecular structure indicating ferromagnetism or antiferromagnetism. Thus, the present inventor noted a dinuclear complex (metal complex which has two metal atoms as nuclei). This is because ferromagnetic or antiferromagnetic interaction is often observed in the dinuclear complex including a paramagnetism metal ion. When heavy metal is selected as the metal atom, the phosphorescence light emission can be further promoted since the heavy atom effect is also added.
Thus, particularly, a metal complex, in which the metal atom shown in FIGS. 1A and 1B substitutes for a dinuclear structure composed of two metal atoms (FIGS. 2A and 2B), is more preferable. In this case, a form as shown in FIG. 2A in which sites 201a with the dinuclear structure are located in lattice points and cross-linked through ligands 202a (hereinafter referred to as xe2x80x9ca lattice A2xe2x80x9d) and a form as shown in FIG. 2B in which ligands 202b are located in lattice points and cross-linked through sites 201b with the dinuclear structure (hereinafter referred to as xe2x80x9ca lattice B2xe2x80x9d) are considered. Note that, it is considered that a shape of the lattice is not limited to a quadrangle as shown in FIGS. 2A and 2B and various a polygon (such as a hexagon) can be used. Also, both a two dimensional (plane) structure and a three dimensional (solid) structure can be used for the lattice.
When the metal complex with such a lattice structure is mixed with light emitting material 203 with a suitable molecular size at the synthesis or at the film formation, the light emitting material can be introduced in the positions between the lattices of the lattice structure.
Further, when the metal complex with the dinuclear structure is used for the present invention, there is an advantage that an ordered structure such as a cubit lattice and a tetragonal lattice can be obtained relatively easily since it is easy to form the lattice in a rectangular shape. Therefore, as described above, since the pores are systematically arranged to becomes a state such as a pseudo superlattice structure, there is a possibility that a stable molecular exciton is produced and the improvement of a light emitting characteristic is led.
Thus, according to the present invention, a light emitting device which has an organic EL element including a light emitting layer made of an organic compound in which an EL is obtained and a metal complex with a dinuclear structure of two metal atoms as nuclei, is characterized in that the metal complex has a lattice structure in which sites with the dinuclear structure and ligands are alternately arranged, and the lattice structure has a structure that the sites with the dinuclear structure are located in lattice points and the lattice points are cross-linked through the ligands (lattice A2).
Also, according to the present invention, a light emitting device which has an organic EL element including a light emitting layer made of an organic compound in which an EL is obtained and a metal complex with a dinuclear structure of two metal atoms as nuclei, is characterized in that the metal complex has a lattice structure in which sites with the dinuclear structure and ligands are alternately arranged, and the lattice structure has a structure that the ligands are located in lattice points and the sites with the lattice points are cross-linked through the sites with the dinuclear structure (lattice B2).
Note that, since, as metallic species, metal elements of a group 5 to a group 11 among the transition series elements of Periodic table are easy to form the dinuclear structure, those are suitable for the present invention. Especially, niobium, tantalum, molybdenum, and tungsten are further suitable since they have a larger atomic number than bromine in which the heavy atom effect is clearly produced, and are low cost materials in the transition series elements.
Thus, according to the present invention, a light emitting device which has an organic EL element including a light emitting layer made of an organic compound in which an EL is obtained and a metal complex with a dinuclear structure which has two metal atoms as nuclei is characterized in that the metal complex has a lattice structure of the lattice A2 or the lattice B2 in which sites with the dinuclear structure and ligands are alternately arranged and the metal atoms are made of one element of group 5 elements to group 11 elements of Periodic table.
When the present invention as described above is embodied, a light emitting device which is light, has low consumption power, and which indicates various light emission colors, can be provided at a low cost. Further, an electrical device which is light, has low consumption power, indicates various light emission colors, and which has a low cost, can be provided by using such a light emitting device.